The present invention relates to engraving machines and, more particularly, to a circuit arrangement for canceling the effects of oscillation in an electromechanical engraving system.
The basic principle of electro-mechanical engraving of a gravure cylinder involves rotating a copper plated cylinder while actuating an electrically driven tool which cuts or engraves cells or lines into the copper surface. The engraved cylinder is normally used in a web type gravure printing press for printing paper, plastic, or metallic film material.
In the gravure printing process, the engraved cylinder is flooded with ink. A doctor blade wipes off excess ink from the surface so that only the engraved cells contain ink which is transferred to the material being printed. To obtain a high quality print, it is necessary that the cells be very accurately placed or located on the cylinder surface, usually within one to two microns of the desired theoretical location. The depth of the engraved cells must also be accurately controlled since the depth determines the amount of ink transferred which, in turn, determines the shade of gray in a black-white print. In a color print, the amount of ink transferred to the paper or material is even more critical, since colors are mixed to produce various shades of all possible colors. A slight variation in the desired amount of ink affects not only the darkness of the color but, more importantly, the production of the desired color tone.
The cutting tool used to engrave the cells is normally a pointed diamond stylus. The tool must make many cells in a cylinder and, therefore, must be operated at a very high speed. For example, in a typical 140 line screen, about 20,000 cells per square inch are required. More than 100 million cells are frequently required for a single gravure printing cylinder. Even with a forming rate of 3,000 to 4,000 cells per second, several hours of time may be required to engrave a single cylinder. Such a high cell forming rate introduces serious problems of high acceleration forces with resulting torsional and transverse or lateral vibrations. It is also necessary to make rapid transfer from black to white (full cells to no cells) or white to black. This also introduces transients causing serious torsional and transverse vibrations.
One problem with electromechanical engraving systems, then, is that there is a strong tendency for the mechanical stylus holder and its spring support system to "ring" or vibrate, thereby producing "ghost" images which are displaced from the main image. This greatly reduces the quality of the printing, especially for small type, and dampening means must be used in a form which does not introduce hysteresis.
Electromechanical systems used in engraving machines are subject to step current or torque response, and the systems experience vibrations. The resonant characteristics of an engraving system and the non-linearities are associated with many factors. Changes in the copper hardness, which is the media on which the engraving is normally done, magnetic saturation, movements in non-intended directions of displacement, non-linearities in the materials composing the systems, and excitations upon the impact of the solid diamond or other cutting devices on the engraving head with a solid media on which engraving is achieved, are all factors which contribute to undesired vibrations. Generally, the linear oscillations are predictable from the system characteristics including spring constants, damping coefficients, inertias, and torques. Notch filters and damping techniques are effective in reducing these oscillations. Characteristics of all dampening materials change as they absorb energy due to self-heating. They are therefore inherently unstable in their dampening characteristics. In more complex systems with multiple resonant points in association with other non-linearities, filtering and damping techniques alone are not sufficient and, in many cases, the response of the system or the rise time will suffer.
It is therefore highly desirable and an object of the present invention to provide a system and technique to eliminate mechanical ring or vibrations in electromechanical systems used in engraving machines.